There are, currently, no accepted sensitive and specific biological markersof total dietary fat intake for use in epidemiologic studies. By examining a combiantion of fatty acids, found in low concentrations, reflecting various components of a low fat diet, it may be possible to develop a marker of fat intake. This was tested in a 7 week controlled dietary intervention in 61 post menopausal women, randomized to either a 17% or 34 % fat calories diet. There were highly significant (P<0.001) increases in relative proportions of many erythrocyte and/or plasma phospholipid fatty acids which differentiated the two groups very well. These findings are highly promising but there a number issues which need to be resolved. We propose to extend the study to other populations while looking at fatty acid changes over time, weight loss and body composition.